Electron Affinity Values — First Electron Affinities of the Elements
| Element | Symbol | Z | Electron Affinity (kJ/mol) | Group | Period |
|---|---|---|---|---|---|
| Hydrogen | H | 1 | -73 | 1 | 1 |
| Helium | He | 2 | 21 | 18 | 1 |
| Lithium | Li | 3 | -60 | 1 | 2 |
| Beryllium | Be | 4 | 18 | 2 | 2 |
| Boron | B | 5 | -27 | 13 | 2 |
| Carbon | C | 6 | -122 | 14 | 2 |
| Nitrogen | N | 7 | 7 | 15 | 2 |
| Oxygen | O | 8 | -141 | 16 | 2 |
| Fluorine | F | 9 | -328 | 17 | 2 |
| Neon | Ne | 10 | 29 | 18 | 2 |
| Sodium | Na | 11 | -53 | 1 | 3 |
| Magnesium | Mg | 12 | 21 | 2 | 3 |
| Aluminum | Al | 13 | -42 | 13 | 3 |
| Silicon | Si | 14 | -134 | 14 | 3 |
| Phosphorus | P | 15 | -72 | 15 | 3 |
| Sulfur | S | 16 | -200 | 16 | 3 |
| Chlorine | Cl | 17 | -349 | 17 | 3 |
| Argon | Ar | 18 | 35 | 18 | 3 |
| Potassium | K | 19 | -48 | 1 | 4 |
| Calcium | Ca | 20 | -2 | 2 | 4 |
| Gallium | Ga | 31 | -41 | 13 | 4 |
| Germanium | Ge | 32 | -119 | 14 | 4 |
| Arsenic | As | 33 | -78 | 15 | 4 |
| Selenium | Se | 34 | -195 | 16 | 4 |
| Bromine | Br | 35 | -325 | 17 | 4 |
| Krypton | Kr | 36 | 39 | 18 | 4 |
| Iodine | I | 53 | -295 | 17 | 5 |
| Xenon | Xe | 54 | 41 | 18 | 5 |
| Gold | Au | 79 | -223 | 11 | 6 |
| Astatine | At | 85 | -270 | 17 | 6 |
Electron affinity is the enthalpy change for X(g) + e- → X-(g). Sign convention here is thermodynamic: negative = exothermic = favorable. Some older texts (especially pre-1990s) report the magnitude as a positive number with the opposite sign convention — verify which one your problem set is using before computing. Noble gases and most Group 2 elements have positive (unfavorable) values because the added electron would have to enter a higher-energy shell or subshell. Chlorine, not fluorine, has the most negative first EA among neutral atoms (-349 vs -328 kJ/mol); fluorine's tiny 2p shell forces unfavorable electron-electron repulsion onto the incoming electron. Sources: NIST, CRC Handbook.
Frequently Asked Questions
Why does chlorine have a more negative electron affinity than fluorine?
Looks like a trend violation, but it's actually clean physics. Fluorine's 2p subshell is so small that the existing seven 2p electrons already sit close together and repel each other strongly; an incoming eighth electron pays an extra penalty for joining that crowded shell. Chlorine's 3p subshell is roomier, so the same +1-charge attraction wins out without the same repulsion penalty. Result: Cl releases 349 kJ/mol on accepting an electron, F only 328. The same pattern appears one row down — sulfur has a more negative EA than oxygen for the same reason.
Why do noble gases have positive electron affinities?
A noble gas already has a closed-shell configuration, so an extra electron has nowhere efficient to go — it has to start the next principal quantum shell. Adding an electron to neon, for example, would mean populating a 3s orbital that's much higher in energy than the filled 2p. That costs energy rather than releasing it, hence the positive EA. The hypothetical Ne- anion is also unbound — it falls apart back to neutral Ne plus a free electron. The same applies up and down Group 18 and (in attenuated form) to Group 2, where the 2s subshell is also full.
How does electron affinity relate to electronegativity?
Two related but distinct quantities. Electron affinity is the experimentally measured energy change when a free gas-phase atom captures an electron — a single hard number in kJ/mol. Electronegativity is the tendency of an atom in a bond to pull shared electron density toward itself, expressed on an arbitrary scale (Pauling, Mulliken, Allen). Mulliken electronegativity is literally defined as the average of ionization energy and electron affinity, which makes the link explicit. The correlation with EA is strong but imperfect — fluorine is the most electronegative atom but chlorine has the more negative EA, exactly because EA also reflects shell-size repulsion effects.